Vapor turbines



June 26, 1962 H. z. TABOR ET AL VAPOR TURBINES Filed March 21, 1960 su.s

ite rates 3,040,528 VAPOR TURBINES Harry Zvi Tabor, Beth Hakercm,Jerusalem, and Lucien Bronicki, Katam'on, Jerusalem, Israel, assignorsto the State of israel Filed Mar. 21, 1960, Ser. No. 16,256 Claimspriority, application Israel Mar. 22, 1959 1 Claim. (Cl. 60-36) dS ar@(The boundary in question will herein be referred to as vapor/liquidboundary although in fact it is a boundary between the state in whichthe liquid andthe vapor coexist and a state in which there exists onlyvapor.) This means that when saturated steam expands isentropically theeiiluent is within the region in which steam and liquid water `co-exist.In other words, the eiuent steam is not superheated and the theoreticalefficiency of a Rankine cycle, carried out Ywith steam, is accordinglysatisfactory. This, however, entails the result Athat in a steamturbine, particularly a small single-stage turbine, the steam, saturatedat the beginning, is wet afterexpanf sion in the nozzle. IIf thiswetness exceeds a .certain amount it can cause erosion of the turbineblades, which is a serious disadvantage.

A further disadvantage of the use of steam `resides in the fact thateven for moderate enthalpy (total heat) drops the etilux velocity fromthe nozzles is very high, making it very diflicult or impossible tooperate a singlewheel turbine at the correct speed which, as known, hasto` be about half the etliux velocity. In addition it is known that forsmall turbines, Le. for low-HP. turbines, the blades and the nozzle mustbe extremely small,

which is another source of considerable ineliiciency sincer experienceshows that the eiiiciency of a turbine stage decreases rapidly as thesize of the blades is reduced.

The elux velocityof a vapor (for expansion between two lixedtemperatures) is to a tirst approximation inversely proportional to thesquare root of the molecular weight, or, in other words, the higher themolecular weight of the iluid the smaller the eiiiux velocity.Hereinafter fluids of higher molecular weight will be referred to asheavy Because of the above approximate relation between the efflux'velocity and the molecular weight, it has frequently been suggested touse heavy vapors for the operation of turbines. The use of such heavyvapors would indeed remove the above-mentioned disadvantages inherent insteam. Thus, for example, a vapor nine times as heavy as steam, i.e. ofmolecularV Weight of 162, will give eliiux velocities of the order ofone-third of that of steam so that a single turbine stage will sutlicein many cases or, in the case of a large number of stages, the numbermay be reduced to one-ninth (pressure staging) or to a third (velocitycompounding) 4for the same speed and diameter of the rotor. Anotheradvantage of theV use of heavy fluids results from thevfact that for thesame output a `larger mass of vapor must. pass through the turbine as aresult of which the size of the blades and with it the efficiency of theturbine is increased. Y

From the above it becomes apparent that for small turbines where onewishes to avoid the complication of multiple stages, or even Velocitycompounding, and at the same time -to increase the size of the nozzlesand blades for better efficiency the use of a heavy vapor isadvantageous as compared with steam. Furthermore, if the boiling pointof the vapor is high, i.e. the density of the vapor in which the bladesrotate is lower than for steam, the disc and bucket friction losses fora given speed can be reduced below those for steam.

However, in the use of heavy iluids for the operation of a turbine aninherent diiiiculty is encountered resulting from the fact that theslope of the liquid/vapor boundary of the temperature-entropy diagram offluids usually changes from negative to positive as the number of atomsinthe molecule is increased, i.e.

becomes greater than zero. The implication ot this will now be explainedwith reference to the drawing in which: FIGS. l and 2 aretemperature-entropy diagrams of water and of a heavy fluid,respectively; FIG. 3 is a diagram-- matic view of an apparatus vforcarrying out the process of the invention,

When carrying out a Rankine cycle with steam starting from point 1, thewater is iirst heated from T1 to T2 (FIG. l) with the entropy beingincreased accordingly. (For the sake of simplicity the compression ofthe liquid is here ignored.) At T2, which corresponds -to point 2 on thecurve, steam is generated isothermically; that part of the cycle isrepresented by the stretch 2, 3. The points 2, 3 lie on the branches of.the diagram marking the water/water-steam-mxture boundary vand thewater-steam-mixture/ steam boundary, respectively. From point 3 the nextstage of the cycle consists in an isentropic expansion which in an idealcase proceeds along the stretch 3, 4. However, in practice the expansionis never quite isentropic and, allowing for frictional losses, a stateis reached 'which is represented by point 4.

In FIG. 2, 1 is again the starting point in the Rankine cycle, thestarting temperature being T1. The liquid is heated to T2 and its stateat this temperature is represented by point 2. The stretch 2, 3represents vaporization. In the case of ideal expansion, the expansionis represented by a line 3, 4 normal to the abscissa; in practice,however, ideal conditions are not realized in a turbine and the path isalong line 3, 4. This means that by carrying out a Rankine cycle with aheavy fluid, expansion yields superheated vapor resulting in lowtheoretical efficiency of the cycle and the necessity for the condensersto desuperheat before condensing.

It is accordingly the object4 of the present invention to devise amethod for overcoming the above'disadvantage inherent in heavy fluids,and thereby to adapt the latter for the operation of turbines based onthe Rankine cycle.

In some cases, however, it also happens that a superheated eiiiuent isobtained with `fluids of which the slope of the temperature-entropydiagram on the liquid/vapor boundary is zero or even negative. This canbe the result of special circumstances such as, for example, 'highfriction inside the turbine. Y

Therefore, in a more general Way it is `the object of the presentinvention to provide a method for the operation of turbines based uponthe Rankine cycle in which the vapors leaving the exhaust are normallysuperheated.

Y The invention consists in a method 4for the operation of a turbinebased on the Rankine cycle and in which the exhaust vapor is in asuperheated state, wherein the Vexhaust vapor is made to give ofi heatto the feed iiuid.v

Finally, a further aspect of the invention consists in apower-generating unit comprising a vapor turbine based upon the Rankinecycle and operable with saturated vapors of such fluids and/or undersuch conditions which yield superheated exhaust vapors, characterized inthat a heat exchanger is provided for transferring the superheat fromthe exhaust vapors to the feed liquid.

According to a preferred embodiment of the invention, the desuperheatingheat exchanger is of the countertlow type. Because of bad heat transferfrom superheated vapors, the heat exchanger has, moreover, to be of aslarge a contact surface as possible and/or reconcilable with otherconsiderations.

Because the heat exchange takes place between the liquid and gaseousphases of one and the same fluid, the vapors can only be cooled down tosaturation in case of an ideal heat exchanger while in practice thevapor leaving the desuperheating heat exchanger will still be slightlysuperheated. For this reason it is in some cases advantageous to inserta cooler between the heat exchanger and the condenser, if any, in orderto economize on the latter.

As pointed out before it may happen owing to special circumstances thatthe exhaust vapors are superheated in the case of fluids in which of theliquid/ vapor boundary of the temperature-entropy diagram is zero oreven negative, while when a fluid is used whose on the liquid/ vaporboundary is positive the exhaust vapor is invariably in a superheatedstate. The invention is applicable in both cases.

Moreover, the invention is applicable to both condensing andnon-condensing turbines, although in most cases, in particular whenfluids other than water are used, condensing turbines are preferred.

The invention is illustrated, by way of example only, in theaccompanying FIG. 2 referred to above and FlG. 3 which is a diagram of aturbine power unit in accordance with the present invention.

From FIG. 2 it can also be seen that, as a result of the exchangebetween superheated vapors and the feed liquid, the former isdesuperheated down to a point S which lies on the temperature-entropydiagram. This desuperheating proceeds theoretically along theconstant-pressure line P1 which is either the pressure inside thecondenser in case of a condensing turbine or atmospheric pressure incase of a non-condensing turbine. However, in a real heat exchanger inwhich there exists a small pressure drop on the vapor side thedesuperheating proceeds along a slightly different path P1 shown as 4S5where 5 is the exit condition from the desuperheating heat exchanger andthe cooling represented by the stretch 55 takes place in the inlet Zoneof the condenser, if any, or in a cooler inserted for the purpose.(Points 5 and 5' approach one another as the temperature differential atthe cold end of the heat exchanger, i.e. between the cooled vapors andthe liquid to be heated, is reduced. Possible pressure differences ofthe vapor at the inlet and at the outlet of the heat exchanger, due tochanges in area of flow, have been ignored for reasons of simplicity.Likewise, compression of the feed liquid has been ignored.) During thedesuperheating of the vapor the quantity of the heat given off is equalto the area 45 C D (in the case of an ideal heat `exchanger). In such anideal heat exchanger this entire heat is given off to the feed liquid sothat the latter is heated from T1 to T3, T3 being defined by the point Bon the lefthand side of the diagram and the latter being determined bythe fact that the areas A B F E and d'5 C D (transferred heat) have tobe equal to each other.

The benefit derived from such a heat exchanger is very i large. Influids such as octane operating over a range of Il C. to 40 C., theamount of energy fed back by this heat exchanger may be of the order of30% of the total energy required by the boiler so that the cycleefficiency will be of the order of 30% more than if the heat exchangerwere omitted.

This method is not to be confused with the regenerative feed heating asused in a steam turbine. In such a system steam is bled off the turbineat various points during the expansion and used to preheat the feed. lnthis manner the steam bled off does not continue through the turbine sothat it does not complete its work possibilities while the throttlesteam flow is increased, thereby increasing the feed-pump work. Contrarythereto the vapors used in accordance with the invention are exhaustvapors, i.e. they are used at a stage at which they already haveperformed their work while they are at the same time in a superheatedstate, i.e. in addition to their unrecuperable latent heat ofcondensation they contain heat which can be recuperated.

A further advantage of the invention is that it allows the frictionalheat produced in the turbine to be saved and fed to the boiler incontrast to the steam cycle where the friction heat is wasted invaporizing some of the wetness in the final steam mixture whichsubsequently has to be condensed anyhow. The system of the invention isparticularly advantageous in a single-stage turbine where normally thereis no reheat factor and where, in accordance with the invention, theemerging friction heat may be a considerable part of the total heatsupply. Even the final kinetic energy of the exhaust-which is usuallycalled exhaust loss-can be caught in the heat exchanger and usefullyemployed to preheat the feed.

It is found in many cases that the efficiency of such a cycle using afluid giving a superheated exhaust and using a good heat exchanger islarger than with a fluid whose vapor after passage through the turbineis wet so that no desuperheating heat exchanger can be used.

FIG. 3 illustrates diagrammatically a unit according to the inventionwhich in this case is a condensing unit. It comprises a turbine 10coupled with an electric power generator l1, a boiler 12, a counterflowheat exchanger 13, a cooler 14, a condenser 15 and a pump 16. The liquidis vaporized in the boiler and the vapors pass into the turbine which isthereby operated. After leaving the exhaust the superheated vapors passthrough the heat exchanger 13 where they give off their superheat to therecycled feed liquid. The de-superheated vapors then enter the cooler 14and from there pass into the condenser 15 where they are liquefied. Theresulting liquid is pumped by means of pump 1 6 through the heatexchanger 13 back into the boiler 12. ln many cases the cooler 14 may beomitted and the cooling of the desuperheated vapors is effected in thefirst part of the condenser l5. Both cooler 14 and condenser l5 arecooled by cooling water or air in any conventional manner.

The choice of the working fluids is quite wide and depends upon theoperating temperatures and the molecular weight deemed necessary. Thusthe higher parafms such as octane (normal or iso) are found to be verysatisfactory as they appear to be very stable in contact with the usualmaterials of construction (iron, aluminum, copper, etc.). The heavieraromatics and ethers are also quite suitable. For very small turbines,where the disc and windage losses can be serious and the nozzles andblades are very small, a higher-boiling liquid is chosen (such asmonochlorobenzene or dichlorohenzene) so as to reduce the losses andincrease the nozzle and blade size. For larger turbines, where the discand windage losses are, in many cases, small in proportion to theoutput, somewhat lower-boiling fluids are chosen (such as isooctane), inorder to avoid having to make nozzles and blades unnecessarily large.

We claim:

Method of generating power, comprising the steps of:

5 (a) circulating 'ina closed circuit a fluid having a molecular Weightgreater than that of water and a Aboiling point at atmospheric pressurein excess of substantially 90 C., said fluid being'se-lected from thegroup which consists of octane, parains having a molecular Weight inexcess of that of octane, and chlorinated benzenes While havingV atemperatureentropy diagram wherein the slope of the line designating theliquid-vapor/vapor boundary is positive; (b) vaporizing said fluid atsaturation temperature; (c) expanding in a turbine the saturated Vaporso produced, thereby superheating said vapor;

-(d) removing enough sensible heat from the resulting superheatedeffluent emanating from the turbine to de-superheat said eiuent;

(e) condensing the de-superheated efliuent to produce aliquid;

(f) preheating said 4liquid with the heat removed from said effluent;and

(g) repeating the cycle by again vaporizing the preheated liquid atsaturation temperature.

References Cited in the file of this patent UNITED STATES PATENTS Delasa Oct. 23, r1.928 Cross et al Nov. 24, 1942 Benning et al May 31, 19494Riehl Apr. 26, 1955 Riehl May 28, 1957 FOREIGN PATENTS n Germany Dec.3, 1914

